1. Field
The present disclosure generally relates to composite structures and, in particular, to the formation of composite structures. Still more particularly, the present disclosure relates to a method and apparatus for producing radius fillers used to fill channels between composite structures.
2. Background
Composite materials are tough, lightweight materials created by combining two or more functional components. For example, a composite material may include reinforcing fibers bound in a polymer resin matrix. The fibers may be unidirectional or may take the form of a woven cloth or fabric. The fibers may be pre-impregnated in a layer of resin, producing a ply of prepreg. In thermoset composites, separate fibers and resins, or prepreg sheets, are arranged and cured to form a structural member.
When composite structural members are joined together, channels or voids may be present between the members. These channels are filled in order to increase the strength of the structural member. For example, in the aircraft industry, some composite stiffeners include a filler at the radius bond line between the stiffener and a skin panel. As an example, a filler may be used at the radius bond line between a stringer and a skin panel.
In some cases, the filler takes the form of a triangular cross-sectional structure which fills the voids between the members. This triangular cross-sectional structure is sometimes referred to as a “radius filler,” a “noodle,” or a “composite filler.”
A radius filler may be formed from composite materials such as adhesive, prepreg tape, fabric, or other types of composite materials. When the filler has a desired level of stiffness, the filler transfers some of the load from the stiffener into the base.
Oftentimes, a radius filler is co-cured with the surrounding composite lamina. In this process, the filler is pre-formed with a desired cross-sectional shape, placed within the channel between structures, and then cured with the surrounding structures. The process of co-curing the radius filler with the surrounding composite lamina causes stresses within the radius filler. In particular, these stresses occur within the radius filler as elements of the part heat up and expand, and following curing, those elements cool down and compress at different rates. For instance, as the surrounding structures cure, those structures serve as a stiff boundary condition for the filler. As a result, the radius filler experiences thermal cure induced stress, which increases a propensity for cracking within the filler.
In some cases, pre-cured radius fillers may be used to reduce the occurrence of thermal induced stresses. With pre-cured radius fillers, the filler is cured, subsequently placed into the channel between structures, and then bonded with the structures as they cure. Using pre-cured radius fillers, however, may add additional time and complexity to the manufacturing process. For example, pre-cured radius fillers may need to be bonded along complicated interfaces or may not have a desired cross-sectional shape. In addition, adding manufacturing steps by pre-curing the filler is not conducive to faster production cycles.
Therefore, it would be desirable to have a method and apparatus that take into account at least some of the issues discussed above, as well as other possible issues. Specifically, one issue is finding a method for curing fillers to reduce thermal induced stresses from co-curing processes.